Sulfur oxides are generally removed from electric power-producing combustion systems in a flue gas desulfurization (FGD) system which scrubs the sulfur oxides, primarily sulfur dioxide, with a limestone slurry. This type of process produces gypsum (CaSO.sub.4.2H.sub.2 O--calcium sulfate dihydrate) for sale or disposal. The scrubbing slurry contains lime or limestone, which is reacted with absorbed sulfur dioxide and oxygen to produce the gypsum. However, these processes can be unreliable because of the presence of suspended solids in the scrubbing solution. The suspended solids can form deposits of solids or scale on the scrubber and other FGD system equipment and also cause abrasion and erosion of the equipment. In addition, high liquid to gas ratios are required due to the limitations of the liquid phase buffering capacity. Also, the addition of packing to the absorber, which would make the scrubbing process more efficient, is not recommended in slurry-based FGD processes due to the potential for the packing to become plugged by the suspended solids and scale formation. The cost of the scrubbing system could be reduced if the plugging, scale formation and equipment abrasion problems could be eliminated. Moreover, cost savings could also be achieved by increasing the alkalinity of the FGD system recirculating liquor so that a lower liquid to gas ratio could be used.
The recirculation of a clear liquor in the scrubbers of FGD systems has been proposed to eliminate the problems accompanying the use of suspended solid slurries and low liquid phase alkalinity in the scrubbers. When an FGD process recirculates clear liquor, the lime or limestone required to scrub the sulfur dioxide is added to the system in a reactor separate from the liquid/gas contactor. One illustrative process, which is widely used, is a dual alkali process, which employs a sodium sulfite clear liquor scrubbing solution. The inventors of the present invention have developed another clear liquor scrubbing system that uses organic acids to provide the required alkalinity under inhibited oxidation conditions. This process is described in U.S. patent application Ser. No. 08/109,487 filed on Aug. 20, 1993, owned by the assignee of the present invention. In all of the known clear liquor scrubbing processes that use lime or limestone for regeneration, a clear scrubbing solution is recirculated in the absorber or scrubber, and a stream of spent liquor is sent to an external reactor for regeneration. Lime or limestone is mixed with this stream to precipitate the absorbed sulfur dioxide. The precipitated solids are separated from the liquor, and the clarified liquor is returned to the absorber to remove more sulfur dioxide frown the flue gas.
U.S. Pat. Nos. 4,080,428 and 4,222,993 to Holter et al. describe flue gas desulfurization processes which employ a clear scrubbing or wash liquor and use organic acids to provide the required alkalinity. Both of these processes require elevated calcium ion concentrations, however, which can adversely affect the FGD process. Further, the limestone reaction rate is reduced so that poor limestone utilization results. Holter et al. U.S. Pat. No. 4,222,993, moreover, defines process parameters, most notably the high pH range, which can only be achieved with lime, which is more expensive than limestone.
While these clear liquor scrubbing systems are effective in sulfur dioxide removal, most of them produce a waste stream of calcium sulfite, which has little, if any, use as a by-product and must therefore be disposed of. Gypsum (CaSO.sub.4.2H.sub.2 O), which has value for producing wallboard, for cement production and as an agricultural soil additive, is the preferred FGD process by-product.
U.S. Pat. No. 4,213,946 to Furuta et al. addresses one of the disadvantages of producing a gypsum by-product in a flue gas desulfurization process. This patent discloses the use of high concentrations of calcium chloride at low calcium hydroxide concentrations to prevent gypsum scale formation. The high calcium concentrations of up to 40% by weight disclosed by Furuta et al., however, are detrimental to scrubber performance in a clear liquor, organic acid-buffered FGD process, since calcium sulfate scale is more likely to be formed in the scrubber at such high calcium concentrations. Further, the FGD process taught by Furuta et al. uses the more expensive alkali calcium hydroxide rather than the less expensive limestone (CaCO.sub.3).
The available clear liquor scrubbing FGD processes, including those described in the Holter et al. patents, produce calcium sulfite. The calcium sulfite can be oxidized to gypsum; however, the production of gypsum in a clear liquor scrubbing process is accompanied by some drawbacks. Gypsum can be produced by oxidizing the calcium sulfite waste stream from the FGD process. This requires lowering the pH of the slurry, usually with sulfuric acid, and oxidizing the calcium sulfite to gypsum by forcing air through the resulting slurry in a relatively tall mixing reactor or column. Because a separate reactor step and additional reagents (sulfuric acid and compressed air) are required, the costs of the FGD system are increased.
Calcium sulfite may also be oxidized in a clear liquor FGD system by forcing air into the bottom of the scrubber or reaction tank as is done in a slurry FGD system. However, in a clear liquor scrubbing system, either the alkalinity of the liquor is lost and/or the scaling potential for gypsum in the scrubber is increased. Low scrubbing pHs are therefore required to maintain the calcium sulfite in solution until it is oxidized. These low scrubbing pHs reduce scrubbing efficiency and may result in high vapor losses and degradation of the organic acid buffer used in clear liquor scrubbing systems. Formate buffers, especially, are adversely affected. Because the low scrubbing pH lowers scrubbing efficiency, high liquid to gas ratios are required to maintain effective scrubbing.
The prior art, therefore, has failed to provide a clear liquor scrubbing, organic acid buffered flue gas desulfurization system or process conducted under forced oxidation conditions which effectively and efficiently scrubs sulfur dioxide from flue gases to produce a usable gypsum by-product without scaling the FGD system components. A need exists for such a system and process.